Effect of Minor Sc Addition on the Microstructure Evolution of Al-Cu-Li-Mg Alloy During Homogenization with Different Cooling Modes

The microstructure evolution and mechanical properties of Al-3.92Cu-1.10Li-0.68Mg-0.32Ag-0.23Mn-0.35Zn-0.11Zr and Al-3.92Cu-1.12Li-0.76Mg-0.29Ag-0.24Mn-0.24Zn-0.12Zr-0.083Sc (wt%) alloys during homogenization with different cooling modes were comprehensively studied. It was clearly revealed that the minor Sc addition refined the grains of as-cast Al-Cu-Li-Mg alloys through the formation of primary Al-3(Sc, Zr) phases during the solidification process, and induced Sc enrichment in the theta (Al2Cu) eutectic. After the homogenization treatment, the majority of coarse nonequilibrium eutectic phases at the grain boundaries dissolved into the alpha-Al matrix, leading to the relatively uniform distribution of each element. Sc-added alloy after homogenization treatments possessed more superior strength and ductility than Sc-free alloy. Compared to the furnace cooling mode, the air cooling mode could inhibit the precipitation of micron-sized coarse T-1 (Al2CuLi) phase and improve the mechanical properties of the alloys. After homogenization, the continuous Al2Cu phase in the as-cast Sc-added alloy dissolved and transformed to an array of W-(Al, Cu, Sc) ternary phase, which was identified as Al6Cu6Sc with the body-centered tetragonal structure. The appearance of spherical Al6Cu6Sc particle could not damage the mechanical properties of Al-Cu-Li-Mg alloys after the homogenization treatment. The tensile strength, yield strength and elongation of Sc-added alloy after the homogenization by air cooling were 390 MPa, 265 MPa and 10.8%, respectively. The investigation of Al6Cu6Sc phase offered a potential avenue to produce high-quality Sc-added 2xxx series alloys.